At least temporarily, Dr. Reuben has abandoned screwing and turned his attention to eating. The result is one more near-miraculous prescription for dieting your way to protection against cancer, heart disease, diverticulosis, hemorrhoids, varicose veins, phlebitis and obesity. The secret, Dr. Reuben confides, is roughage--the coarse vegetable fibers which aid and speed digestion: the more you ingest the bulkier, softer and more odor-free will be your bowel movements. Rural Africans on a high-roughage diet eliminate four times as much fecal matter as Englishmen and Americans, and far more quickly. This keeps carcinogens out of prolonged contact with the large intestine and provides ""almost total immunity""against cancer of the colon or rectum as well as helping the elimination of cholesterol and ""most expert cardiologists agree that cholesterol is the primary cause of heart attacks."" (""Mostcardiologists"" do not agree; and Dr. Reuben wholly disregards stress, high blood pressure and other causative factors.) The dietary recommendations are unobjectionable--cut down on processed sugars and over-refined carbohydrates and eat whole grains with a daily supplement of bran. Dr. Reuben's wife, Barbara, contributes some high roughage recipes and the doctor supplies copious footnotes to support his probably innocuous, but surely exaggerated, claims for the prophylactic effects of a good bowel movement.